1. Field of the Invention
The present invention relates to a printing system for recording or erasing a visible image on/from a recording medium, where the state of the recording medium is changed to a cloudy state or a transparent state by applying heat energy thereto. The visible image can be repeatedly recorded or erased using a thermal recording device such as a thermal head.
2. Description of the Related Art
In the conventional hard copy, an image is formed on a recording medium such as paper by using an image forming material such as ink or toner provided from an external unit. Alternatively, a recording layer may be formed on a base material such as thermal recording paper, and a visible image may be formed on the recording layer. In other words, a permanent image was recorded.
However, in recent years, in accordance with the spread of the formation of various networks, facsimiles, and copy machines, the consumption of these recording mediums is rapidly increased, there occurs a problem of natural destruction such as deforestation and a social problem such as refuse disposal or rubbish. In order to deal with these problems, the reduction of the consumption of the recording medium such as recycle of the recording paper has been required. Along the lines, a recording medium, which can record/erase repeatedly, has recently been developed.
As a recording medium having such a characteristic, for example, Published Unexamined Japanese Patent Application No. 55-154198 discloses a recording medium, which can reversibly change between the transparent and cloudy states according to the temperature of the recording material.
In this recording medium, for example, if the temperature is increased to a first threshold temperature from a normal cloudy state, the recording medium is changed to a transparent state from the cloudy state, where the transparent state is maintained after the temperature returns to the normal temperature. If the temperature exceeds the first threshold temperature and is increased to a second threshold temperature, the recording medium is in a cloudy state, where the cloudy state can be maintained even after the temperature is returned to the normal temperature. This change can be repeatedly reproduced.
The discussion on the deterioration of resolution occurred when an image is repeatedly recorded on and erased from such a recording medium is reported in e.g., Proceedings of 4th Japanese Symposium on Non-impact Printing Technologies Symposium, 3-2, p 57 (1987).
Moreover, as disclosed in Published Unexamined Japanese Utility Model Application No. 2-19568, there has been proposed a display changing apparatus for displaying and erasing using a display medium having a heat reversible recording material. This apparatus comprises erasing means for thermally erasing characters on the display medium and printing means for thermally printing the characters. As a specific example, this document describes the structure in which the heat reversible display on the display medium for a floppy disk cartridge is erased by a heater head (erasing means), and displayed or written by a moving thermal head (printing means).
Furthermore, Published Unexamined Japanese Utility Model Application No. 2-3876 discloses an apparatus for recording/erasing a display on/from a data recording card having a heat reversible recording layer by use of a heat roller (erasing means) and a thermal head (printing means).
Moreover, in Japan Hardcopy '90, NIP-2, p 147 (1990), there has been reported a recording material using leuco dye, as a coloring source, which can provide a reversible tone change by based on the thermal energy.
As mentioned above, the printing system using the recording medium, which can repeatedly record and erase, can solve the problems of the conventional printing system. Particularly, a recording medium, which is formed of a compound recording material layer of low/high polymers repeatedly showing the cloudy and transparent states according to the above-mentioned different temperature process, is an excellent material, for recording and erasing using the thermal head frequently employed in the conventional thermal recording means.
Conventionally, when a visible image is recorded or erased to/from the recording medium repeatedly showing the cloudy and transparent states by the above-mentioned different temperature process using the thermal head as thermal recording means, the recording or erasing is performed by voltage-amplitude-modulating or pulse-number-modulating a driving pulse for electrically driving a heating resistance member (heating element) of the thermal head. In such a conventional apparatus, duty factor of the driving pulse at the time of recording and erasing is the same. However, according to the conventional printing system using such a recording medium, the relationship between the recording temperature and the erasing temperature is:
recording temperature (or cloud temperature)&gt;erasing temperature (or transparency temperature).
Therefore, there was a problem in that the temperature control range for erasing was extremely small and recording or erasing of the stable visible image was not able to be performed.